Light detection device

ABSTRACT

A light detection device includes an APD, a plurality of temperature compensation diodes, and a circuit unit. The plurality of temperature compensation diodes have different breakdown voltages lower than a breakdown voltage of the APD. The circuit unit puts any one of the plurality of temperature compensation diodes into a breakdown state. The circuit unit includes a plurality of terminals and a terminal. The plurality of terminals are respectively connected to electrodes of the mutually different temperature compensation diodes. The terminal is electrically connected to the APD and electrodes of the temperature compensation diodes.

TECHNICAL FIELD

The present invention relates to a light detection device.

BACKGROUND ART

A configuration is known in which a bias voltage applied to an avalanchephotodiode is controlled in order to provide stable light detection withrespect to temperature (for example, Patent Literature 1). In PatentLiterature 1, a voltage corresponding to the breakdown voltage of atemperature compensation diode is applied to the avalanche photodiode asa bias voltage. Hereinafter, in this specification, the “avalanchephotodiode” will be referred to as an “APD”.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication No.H07-27607

SUMMARY OF INVENTION Technical Problem

In a light detection device, it is excepted to obtain a desired gain ina stable manner with respect to temperature in the APD. However, thegain of the APD changes according to the change in the bias voltageapplied to the APD. Even if a constant bias voltage is applied to theAPD, the gain of the APD changes as the ambient temperature changes.Therefore, maintaining the gain of the APD constant needs to change thebias voltage applied to the APD according to the ambient temperature.

When the difference voltage between the breakdown voltage of the APD andthe bias voltage applied to the APD is controlled to be constant, thechange in the gain of the APD is small even if the ambient temperaturechanges. However, since the breakdown voltage of the APD also changesaccording to the ambient temperature, it has been very difficult toswitch the gain so as to obtain a gain desired according to thesituation.

An object of one aspect of the present invention is to provide a lightdetection device capable of obtaining, in an APD, a gain desiredaccording to a situation in a stable manner with respect to temperature.

Solution to Problem

A light detection device according to one aspect of the presentinvention includes an APD, a plurality of temperature compensationdiodes, and a circuit unit. Each of the plurality of temperaturecompensation diodes includes first and second electrodes. The pluralityof temperature compensation diodes have mutually different breakdownvoltages lower than a breakdown voltage of the APD. The circuit unit isconfigured to cause any one of the plurality of temperature compensationdiodes into a breakdown state. The circuit unit includes a plurality offirst terminals and a second terminal. The plurality of first terminalsare respectively connected to second electrodes of the temperaturecompensation diodes different from each other. The second terminal iselectrically connected to the APD and the first electrode of each of thetemperature compensation diodes.

In the one aspect described above, the circuit unit electricallyconnects the APD and each temperature compensation diode to the secondterminal in parallel with each other. In this configuration, when anyone of the plurality of temperature compensation diodes is in abreakdown state, a voltage corresponding to the breakdown voltage of thetemperature compensation diode in the breakdown state is applied to theAPD as a bias voltage. As a result, a difference voltage between thebreakdown voltage of the APD and the bias voltage applied to the APD isset, and the APD has a gain corresponding to the difference voltage.Therefore, according to a temperature compensation diode that breaksdown, a gain desired according to the situation in the APD can beobtained in a stable manner with respect to the temperature.

In one of the above aspects, the circuit unit may include at least oneswitch. At least one switch may be electrically connected to acorresponding temperature compensation diode. At least one switch may beconfigured to switch between a state capable of electrically energizingthe corresponding temperature compensation diode and a state incapableof electrically energizing the corresponding temperature compensationdiode. The plurality of temperature compensation diodes may include afirst temperature compensation diode and a second temperaturecompensation diode. The second temperature compensation diode may have ahigher breakdown voltage than the first temperature compensation diode.The switch may be electrically connected to the first temperaturecompensation diode. In this case, when the first temperaturecompensation diode is set by the switch to a state capable of beingelectrically energized, the first temperature compensation diodepreferentially breaks down even if the second temperature compensationdiode is in a state capable of being electrically energized. Therefore,the breakdown voltage applied to the APD can be selected from the twobreakdown voltages simply by switching the switch electrically connectedto the first temperature compensation diode. Accordingly, it is possibleto switch a gain desired according to the situation in the APD withsimple control.

In the one aspect described above, the at least one switch may beconnected to a corresponding first terminal A high voltage is appliedbetween the first electrode of each temperature compensation diode andthe APD. Therefore, a control in a case where the switch is electricallyconnected to the second electrode through the first terminal can beeasier than that in a case where the switch is disposed between thefirst electrode and the APD.

In the one aspect described above, the circuit unit may be configured tobe set to a state capable of electrically energizing the secondtemperature compensation diode regardless of whether or not to becapable of electrically energizing the first temperature compensationdiode. In this case, even if the first temperature compensation diode isdamaged or a local temperature change occurs in the vicinity where thefirst temperature compensation diode is disposed, the second temperaturecompensation diode breaks down. Therefore, the flow of a large currentto the APD is prevented, and a failure of the light detection device isprevented.

In the one aspect described above, the plurality of temperaturecompensation diodes may further include a third temperature compensationdiode. The third temperature compensation diode may have a breakdownvoltage higher than a breakdown voltage of the first temperaturecompensation diode and lower than a breakdown voltage of the secondtemperature compensation diode. The switch may be electrically connectedto the third temperature compensation diode. The circuit unit may beconfigured to switch, by the switch, between a state capable ofelectrically energizing the third temperature compensation diode and astate incapable of electrically energizing the third temperaturecompensation diode, in a state incapable of electrically energizing thefirst temperature compensation diode. In this case, in a state capableof electrically energizing the first temperature compensation diode, thefirst temperature compensation diode breaks down. In a state incapableof electrically energizing the first temperature compensation diode, thethird temperature compensation diode breaks down when the thirdtemperature compensation diode is set in a state capable of beingelectrically energized. In a state incapable of electrically energizingthe first temperature compensation diode, the second temperaturecompensation diode breaks down when the third temperature compensationdiode is set in a state incapable of being electrically energized. Inthis manner, it is possible to switch a gain desired according to thesituation in the APD with simple control.

Advantageous Effects of Invention

According to one aspect of the present invention, it is possible toprovide a light detection device capable of obtaining, in an APD, a gaindesired according to a situation in a stable manner with respect totemperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a light detection device according to thepresent embodiment.

FIG. 2 is a schematic configuration diagram of the light detectiondevice.

FIG. 3 is a schematic cross-sectional view of a light detection unit.

FIG. 4 is a graph of data indicating the relationship between a biasvoltage applied to an APD and the gain of the APD to which the biasvoltage is applied.

FIG. 5 is a graph illustrating the temperature dependence of the slopeand intercept of the regression line.

FIG. 6 is a graph illustrating the output characteristics of an APDaccording to the setting by a setting unit.

FIG. 7 is a flowchart illustrating a semiconductor substratemanufacturing method.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying diagrams. In addition, in thedescription, the same elements or elements having the same function aredenoted by the same reference numerals, and repeated descriptionsthereof will be omitted.

First, an outline of a light detection device according to the presentembodiment will be described with reference to FIG. 1. FIG. 1 is a blockdiagram of a light detection device. As illustrated in FIG. 1, a lightdetection device 1 includes a detection operation unit 2, a circuit unit3, and a power supply unit 4.

The detection operation unit 2 includes a light receiving unit 10 and atemperature compensation unit 15. The light receiving unit 10 includesat least one APD. In the present embodiment, the APD of the lightreceiving unit 10 is an avalanche photodiode arranged to operate inlinear mode. The temperature compensation unit 15 is configured toprovide temperature compensation for the gain in the APD of the lightreceiving unit 10. The temperature compensation unit 15 is configured tocontrol a bias voltage applied to the APD of the light receiving unit10. The temperature compensation unit 15 includes a plurality oftemperature compensation diodes.

The circuit unit 3 applies a voltage to the light receiving unit 10 andthe temperature compensation unit 15 of the detection operation unit 2.The circuit unit 3 is electrically connected to each electrode of theAPD of the light receiving unit 10 and the temperature compensationdiode of the temperature compensation unit 15. In the presentembodiment, the circuit unit 3 applies, to the APD of the lightreceiving unit 10, a voltage which causes the temperature compensationdiode included in the temperature compensation unit 15 to break down.

The power supply unit 4 generates an electromotive force for operatingthe detection operation unit 2. The power supply unit 4 applies, throughthe circuit unit 3, a potential to the APD of the light receiving unit10 and the temperature compensation diode of the temperaturecompensation unit 15 in the detection operation unit 2. The power supplyunit 4 causes the temperature compensation diode included in thetemperature compensation unit 15 to break down.

By applying a breakdown voltage to any one of the temperaturecompensation diodes of the temperature compensation unit 15, a voltagecorresponding to the breakdown voltage is applied to the APD of thelight receiving unit 10 as a bias voltage. The temperature compensationdiode and the APD have the same temperature characteristics with respectto the relationship between the gain and the bias voltage. In this case,when the ambient temperature changes, the breakdown voltage applied tothe temperature compensation diode changes. Due to the change in thebreakdown voltage applied to the temperature compensation diode, thebias voltage applied to the APD also changes according to the ambienttemperature so that the gain of the APD is maintained. That is, thetemperature compensation unit 15 provides temperature compensation forthe gain in the APD of the light receiving unit 10.

Next, an example of the physical configuration of the light detectiondevice 1 will be described in more detail with reference to FIG. 2. FIG.2 is a schematic configuration diagram of a light detection device. Thelight detection device 1 includes a light detection unit 20, anelectromotive force generation unit 31, a current limiting unit 32, abias voltage stabilization unit 33, and a setting unit 40. The lightdetection unit 20 includes the light receiving unit 10 and thetemperature compensation unit 15 described above. The electromotiveforce generation unit 31 generates an electromotive force for operatingthe light detection unit 20. The current limiting unit 32 limits acurrent flowing through the light detection unit 20. The bias voltagestabilization unit 33 enables a current output equal to or greater thanan upper limit value limited by the current limiting unit 32. Thesetting unit 40 controls the operation of the light detection unit 20. Apart of the light detection unit 20 is included in the detectionoperation unit 2. A part of the light detection unit 20, the biasvoltage stabilization unit 33, and the setting unit 40 are included inthe circuit unit 3. The electromotive force generation unit 31 and thecurrent limiting unit 32 are included in the power supply unit 4.

As illustrated in FIG. 2, the light detection unit 20 includes, inaddition to an APD 11 and the temperature compensation unit 15, a wiringunit 21 for electrically connecting the temperature compensation unit 15and the APD 11 to each other and a plurality of terminals 22, 23, 24,and 25. For example, the terminal 22 is a second terminal, and theplurality of terminals 25 are a plurality of first terminals. In thisspecification, “electrically connects” and “electrically connected” alsoinclude a configuration in which the path is temporarily cut by a switchor the like. In the present embodiment, the temperature compensationunit 15 includes three temperature compensation diodes 26, 27, and 28 asa plurality of temperature compensation diodes described above. Thetemperature compensation unit 15 may include four or more temperaturecompensation diodes.

The APD 11 and the temperature compensation diodes 26, 27, and 28 areincluded in the detection operation unit 2. The wiring unit 21 and theplurality of terminals 22, 23, 24, and 25 are included in the circuitunit 3. The APD 11 includes a pair of electrodes 19 a and 19 b. Each ofthe temperature compensation diodes 26, 27, and 28 includes a pair ofelectrodes 29 a, 29 b. For example, when the electrode 29 a is a firstelectrode, the electrode 29 b is a second electrode. For example, thetemperature compensation diode 28 is a first temperature compensationdiode, the temperature compensation diode 26 is a second temperaturecompensation diode, and the temperature compensation diode 27 is a thirdtemperature compensation diode.

The temperature compensation diodes 26, 27, and 28 break down atmutually different voltages under the same ambient temperature.Hereinafter, a voltage applied to the corresponding temperaturecompensation diode when the temperature compensation diodes 26, 27, and28 break down and a voltage applied to the APD 11 when the APD 11 breaksdown are referred to as “breakdown voltages”. In the followingdescription, when comparing breakdown voltages, it is assumed thatbreakdown voltages at the same ambient temperature are compared witheach other.

The plurality of temperature compensation diodes 26, 27, and 28 havemutually different breakdown voltages. The temperature compensationdiode 26 has a breakdown voltage higher than that of the temperaturecompensation diode 27. The temperature compensation diode 27 has abreakdown voltage lower than that of the temperature compensation diode26 and higher than that of the temperature compensation diode 28. Thetemperature compensation diode 28 has a breakdown voltage lower thanthose of the temperature compensation diodes 26 and 27. The breakdownvoltages of the plurality of temperature compensation diodes 26, 27, and28 are lower than the breakdown voltage of the APD 11.

The wiring unit 21 connects the electrode 19 a of the APD 11, theelectrode 29 a of the temperature compensation diode 26, the electrode29 a of the temperature compensation diode 27, and the electrode 29 a ofthe temperature compensation diode 28 to both the terminal 22 and theterminal 23 in parallel with each other. The wiring unit 21 applies avoltage corresponding to the breakdown voltage of each of thetemperature compensation diodes 26, 27, and 28 to the APD 11 as a biasvoltage.

The terminal 22 is electrically connected to the electrode 19 a of theAPD 11, the electrodes 29 a of the temperature compensation diodes 26,27, and 28, and the current limiting unit 32 of the power supply unit 4.The terminal 23 is electrically connected to the electrode 19 a of theAPD 11, the electrodes 29 a of the temperature compensation diodes 26,27, and 28, and the bias voltage stabilization unit 33. The terminal 24is electrically connected to the electrode 19 b of the APD 11 and asignal reading circuit (not illustrated). The plurality of terminals 25are electrically connected to the electrodes 29 b of the temperaturecompensation diodes 26, 27, and 28 and the setting unit 40. Therespective terminals 25 are connected to the electrodes 29 b of thetemperature compensation diodes 26, 27, and 28 different from eachother. In the present embodiment, the electrode 19 a is the anode of theAPD 11 and the electrode 19 b is the cathode of the APD 11. Theelectrode 29 a is the anode of each of the temperature compensationdiodes 26, 27, and 28, and the electrode 29 b is the cathode of each ofthe temperature compensation diodes 26, 27, and 28.

The electromotive force generation unit 31 and the current limiting unit32 serving as the power supply unit 4 apply a voltage to the lightdetection unit 20. The electromotive force generation unit 31 and thecurrent limiting unit 32 are electrically connected to the terminal 22.In the present embodiment, the positive electrode of the electromotiveforce generation unit 31 is connected to a ground 36, and the negativeelectrode of the electromotive force generation unit 31 is connected tothe terminal 22 through the current limiting unit 32.

The bias voltage stabilization unit 33 increases the upper limit valueof the detection signal output from the APD 11. The bias voltagestabilization unit 33 is connected to the light detection unit 20 andthe electromotive force generation unit 31 in parallel with the currentlimiting unit 32. The bias voltage stabilization unit 33 is, forexample, a capacitor. In the present embodiment, one electrode of thecapacitor is connected to the negative electrode of the electromotiveforce generation unit 31, and the other electrode is connected to theterminal 23. When a pulse signal output from the APD 11 due to incidenceof light is detected, an output having a strength equal to or greaterthan the current value limited by the current limiting unit 32 isobtained according to the capacitance of the capacitor.

The setting unit 40 is configured to set the temperature compensationunit 15 according to the gain to be set in the APD 11. The setting unit40 is configured to select a temperature compensation diode to beoperated among the plurality of temperature compensation diodes 26, 27,and 28. In other words, the setting unit 40 sets a temperaturecompensation diode to be used for controlling the bias voltage among theplurality of temperature compensation diodes 26, 27, and 28. The settingunit 40 sets a temperature compensation diode to be operated bycontrolling the current application states of the plurality oftemperature compensation diodes 26, 27, and 28.

The setting unit 40 includes at least one switch 41. At least one switch41 is connected to a corresponding terminal 25. In the presentembodiment, the setting unit 40 includes two switches 41. One switch 41is electrically connected to the temperature compensation diode 27through the corresponding terminal 25. The other switch 41 iselectrically connected to the temperature compensation diode 28 throughthe corresponding terminal 25. The switches 41 is configured to switchbetween a state capable of electrically energizing correspondingtemperature compensation diodes 27 and 28 and a state incapable ofelectrically energizing the corresponding temperature compensationdiodes 27 and 28. The setting unit 40 controls ON/OFF of the switch 41.

In the present embodiment, the light detection unit 20 includes threeterminals 25. The three terminals 25 are connected to the temperaturecompensation diodes 26, 27, and 28, respectively. The terminal 25connected to the temperature compensation diode 26 is connected to aground 46. The terminal 25 connected to the temperature compensationdiode 27 is connected to a ground 47 through the switch 41. The terminal25 connected to the temperature compensation diode 28 is connected to aground 48 through the switch 41. That is, only one terminal 25 is notconnected to the switch 41. The grounds 46, 47, 48 may be connected toeach other. As a modification example of the present embodiment, theswitches 41 may be connected to all the terminals 25.

Next, the structure of the light detection unit 20 in the lightdetection device 1 will be described in detail with reference to FIG. 3.FIG. 3 is a schematic cross-sectional view of a light detection unit. InFIG. 3, only one of the temperature compensation diodes 26, 27, and 28is illustrated as the temperature compensation unit 15. In the presentembodiment, as illustrated in FIG. 3, the light detection unit 20 is anoptical member including a semiconductor substrate 50. The semiconductorsubstrate 50 has main surfaces 50 a and 50 b facing each other. The APD11 and the temperature compensation diodes 26, 27, and 28 are formed onthe semiconductor substrate 50 so as to be spaced apart from each otherwhen viewed from a direction perpendicular to the main surface 50 a. TheAPD 11 has a light incidence surface 51 a on the main surface 50 a side.The temperature compensation diodes 26, 27, and 28 are light-shieldedAPDs.

The semiconductor substrate 50 includes a semiconductor region 51 andsemiconductor layers 52, 53, 54, and 55. Each of the APD 11 and thetemperature compensation diodes 26, 27, and 28 includes thesemiconductor region 51 and the semiconductor layers 52, 53, and 55.

The semiconductor region 51 and the semiconductor layers 53, 54, and 55are the first conductive type, and the semiconductor layer 52 is thesecond conductive type. Semiconductor impurities are added by, forexample, a diffusion method or an ion implantation method. In thepresent embodiment, the first conductive type is P type and the secondconductive type is N type. When the semiconductor substrate 50 is anSi-based substrate, a Group 13 element such as B is used as the P-typeimpurity, and a Group 15 element such as N, P, or As is used as theN-type impurity.

The semiconductor region 51 is located on the main surface 50 a side ofthe semiconductor substrate 50. The semiconductor region 51 forms a partof the main surface 50 a. The semiconductor region 51 is, for example,P⁻ type.

The semiconductor layer 52 forms a part of the main surface 50 a. Thesemiconductor layer 52 is surrounded by the semiconductor region 51 soas to be in contact with the semiconductor region 51 when viewed fromthe direction perpendicular to the main surface 50 a. The semiconductorlayer 52 is, for example, N⁺ type. In the present embodiment, thesemiconductor layer 52 forms a cathode in each of the APD 11 and thetemperature compensation diodes 26, 27, and 28.

The semiconductor layer 53 is located between the semiconductor region51 and the semiconductor layer 52. In other words, the semiconductorlayer 53 is in contact with the semiconductor layer 52 on the mainsurface 50 a side and is in contact with the semiconductor region 51 onthe main surface 50 b side. The semiconductor layer 53 has a higherimpurity concentration than the semiconductor region 51. Thesemiconductor layer 53 is, for example, P type. In the presentembodiment, the impurity concentration of the semiconductor layer 53 ofeach of the temperature compensation diodes 26, 27, and 28 is higherthan the impurity concentration of the semiconductor layer 53 of the APD11. The semiconductor layer 53 forms an avalanche region in each of theAPD 11 and the temperature compensation diodes 26, 27, and 28.

The impurity concentration of the semiconductor layer 53 of thetemperature compensation diode 27 is higher than the impurityconcentration of the semiconductor layer 53 of the temperaturecompensation diode 26. The impurity concentration of the semiconductorlayer 53 of the temperature compensation diode 28 is higher than theimpurity concentration of the semiconductor layer 53 of the temperaturecompensation diode 27.

The semiconductor layer 54 forms a part of the main surface 50 a. Thesemiconductor layer 54 is surrounded by the semiconductor region 51 soas to be in contact with the semiconductor region 51 when viewed fromthe direction perpendicular to the main surface 50 a. In the presentembodiment, the semiconductor layer 54 has a higher impurityconcentration than the semiconductor region 51 and the semiconductorlayer 53. The semiconductor layer 54 is, for example, P⁺ type. Thesemiconductor layer 54 is connected to the semiconductor layer 55 at aportion that is not illustrated. The semiconductor layer 54 forms ananode of the light detection device 1. The semiconductor layer 54 forms,for example, anodes of the APD 11 and the temperature compensationdiodes 26, 27, and 28.

The semiconductor layer 55 is located closer to the main surface 50 b ofthe semiconductor substrate 50 than the semiconductor region 51. Thesemiconductor layer 55 forms the entire main surface 50 b. Thesemiconductor layer 55 is in contact with the semiconductor region 51 onthe main surface 50 a side. In the present embodiment, the semiconductorlayer 55 has a higher impurity concentration than the semiconductorregion 51 and the semiconductor layer 53. The semiconductor layer 55 is,for example, P⁺ type. The semiconductor layer 55 forms an anode of thelight detection device 1. The semiconductor layer 55 forms, for example,anodes of the APD 11 and the temperature compensation diodes 26, 27, and28.

The light detection device 1 further includes an insulating film 61,electrodes 62, 63, and 64, a passivation film 66, and an antireflectionfilm 67 that are provided on the main surface 50 a of the semiconductorsubstrate 50. The insulating film 61 is stacked on the main surface 50 aof the semiconductor substrate 50. The insulating film 61 is, forexample, a silicon oxide film. Each of the electrodes 62, 63, and 64 isdisposed on the insulating film 61. The passivation film 66 is stackedon the insulating film 61 and the electrodes 62, 63, and 64. Theantireflection film 67 is stacked on the main surface 50 a of thesemiconductor substrate 50.

The electrode 62 penetrates the insulating film 61 to be connected tothe semiconductor layer 52 of the APD 11. A part of the electrode 62 isexposed from the passivation film 66 to form the terminal 24 of the APD11. The electrode 62 outputs a signal from the APD 11 at the terminal24.

The electrode 63 penetrates the insulating film 61 to be connected tothe semiconductor layer 52 of each of the temperature compensationdiodes 26, 27, and 28. A part of the electrode 63 is exposed from thepassivation film 66 to form the terminal 25 of each of the temperaturecompensation diodes 26, 27, and 28.

The electrode 64 penetrates the insulating film 61 to be connected tothe semiconductor layer 54. That is, the electrode 64 is connected tothe APD 11 and the temperature compensation diodes 26, 27, and 28. Inother words, the APD 11 and the temperature compensation diodes 26, 27,and 28 are connected to the electrode 64 in parallel with each other. Apart of the electrode 64 is exposed from the passivation film 66 toform, for example, the terminal 22.

In the present embodiment, the terminal 24 is a pad electrode for thecathode of the APD 11. The terminal 25 is a pad electrode for thecathode of each of the temperature compensation diodes 26, 27, and 28.The terminal 22 is a pad electrode for the anode of each of the APD 11and the temperature compensation diodes 26, 27, and 28.

The APD 11 and the temperature compensation diodes 26, 27, and 28 areconnected to the terminal 22 in parallel with each other. When a reversebias is applied to the APD 11 and the temperature compensation diodes26, 27, and 28, a positive voltage is applied to the pad electrode forthe cathode, and a negative voltage is applied to the pad electrode forthe anode.

The antireflection film 67 is stacked on the semiconductor layer 52 ofthe APD 11. A part of the antireflection film 67 is exposed from thepassivation film 66. Therefore, light transmitted through theantireflection film 67 can enter the semiconductor layer 52 of the APD11. The semiconductor layer 52 of each of the temperature compensationdiodes 26, 27, and 28 is covered with the insulating film 61 and isshielded from light.

Next, the temperature compensation unit 15 will be described in moredetail. Each of the APD 11 and the temperature compensation diodes 26,27, and 28 of the temperature compensation unit 15 have the sametemperature characteristics with respect to the relationship between thegain and the bias voltage. In the light detection device 1, a voltagecorresponding to the breakdown voltage of each of the temperaturecompensation diodes 26, 27, and 28 is applied to the APD 11 as a biasvoltage.

The temperature compensation unit 15 is configured to control the biasvoltage so that the difference voltage between the breakdown voltage ofthe APD 11 and the bias voltage applied to the APD 11 becomes constant.The difference voltage is determined as follows.

Assuming that the bias voltage applied to the APD is “V_(r)” and thegain of the APD to which the bias voltage is applied is “M”, thefollowing equation is satisfied.

[Equation  1]                                      $\begin{matrix}{{\frac{1}{M} \times \frac{dM}{{dV}_{r}}} = {{a \times M} + b}} & (1)\end{matrix}$

“A” and “b” are constants. As can be seen from Equation (1), assumingthat “(1/M)×(dM/dV_(r))” is an objective variable and “M” is anexplanatory variable, for data indicating the relationship between thebias voltage and the gain in the APD, a regression line with a slope of“a” and an intercept of “b” is obtained. As illustrated in FIGS. 4 and5, the slope “a” and the intercept “b” have extremely low temperaturedependence. FIG. 4 is a graph of data indicating the relationshipbetween the bias voltage applied to the APD and the gain of the APD towhich the bias voltage is applied. In FIG. 4, the horizontal axisindicates the gain of the APD, and the vertical axis indicates the valueof “(1/M)×(dM/dV_(r))”. A plurality of lines indicate data of mutuallydifferent ambient temperatures. Specifically, FIG. 4 illustrates data ateight ambient temperatures of 100° C., 80° C., 60° C., 40° C., 20° C.,0° C., −20° C., and −40° C. FIG. 5 is a graph illustrating thetemperature dependence of the obtained slope “a” and intercept “b” ofthe regression line. In FIG. 5, the horizontal axis indicates theambient temperature, and the vertical axis indicates the values of “a”and “b”. The solid line indicates the data of “a”, and the broken lineindicates the data of “b”.

Assuming that the bias voltage applied to the APD is “V_(r)”, the gainof the APD to which the bias voltage is applied is “M”, and thebreakdown voltage of the APD is “V_(br)”, the following equation issatisfied.

[Equation  2]                                      $\begin{matrix}{M = \frac{b\text{/}a}{{\exp\left( {b\left( {V_{br} - V_{r}} \right)} \right)} - 1}} & (2)\end{matrix}$

Here, “a” in Equations (1) and (2) indicates the same physical quantity.“b” in Equations (1) and (2) indicates the same physical quantity.

Therefore, by substituting “a” and “b” obtained from Equation (1) into“a” and “b” in Equation (2), the value of “(V_(br)−Vr)” for the desiredgain is uniquely calculated. “(V_(br)−V_(r))” is a subtraction valueobtained by subtracting the bias voltage applied to the APD from thebreakdown voltage of the APD. That is, “(V_(br)−V_(r))” corresponds tothe difference voltage described above.

Assuming that the difference voltage is “ΔV”, Equation (2) is expressedas Equation (3).

[Equation  3]                                      $\begin{matrix}{{\Delta\; V} = {\frac{1}{b}{\log\left( {\frac{b\text{/}a}{M} + 1} \right)}}} & (3)\end{matrix}$

Therefore, by using Equation (4) in which the gain “M” of the APD inEquation (3) is set to a desired gain “M_(d)”, “ΔV” corresponding to thedesired gain can be easily calculated.

[Equation  4]                                      $\begin{matrix}{{\Delta\; V} = {\frac{1}{b}{\log\left( {\frac{b\text{/}a}{M_{d}} + 1} \right)}}} & (4)\end{matrix}$

Specifically, data indicating the relationship between the bias voltageapplied to the APD and the gain of the APD to which the bias voltage isapplied is acquired at an arbitrary temperature. In the acquired data,the slope of the regression line having “(1/M)×(dM/dV_(r))” as anobjective variable and “M” as an explanatory variable is substitutedinto “a” in Equation (4), the intercept of the regression line issubstituted into “b” in Equation (4), and the desired gain to be set inthe APD 11 is substituted into “M_(d)” in Equation (4). As a result,“ΔV” is calculated. The temperature compensation unit 15 controls thebias voltage applied to the APD 11 so that the difference voltagebecomes the calculated “ΔV”. Here, the acquired data indicating therelationship between the bias voltage and the gain does not have to bethe data of the same APD as the APD 11 as long as the APD has the samematerial and structure as the APD 11.

In the present embodiment, the difference voltage corresponds to asubtraction value obtained by subtracting a voltage corresponding to thebreakdown voltage of each of the temperature compensation diodes 26, 27,and 28 in a breakdown state from the breakdown voltage of the APD 11. Inthe temperature compensation unit 15, a voltage corresponding to thebreakdown voltage of each of the temperature compensation diodes 26, 27,and 28 placed in a breakdown state is applied to the APD 11 as a biasvoltage.

In the present embodiment, the breakdown voltage of the APD 11 and thebreakdown voltage of each of the temperature compensation diodes 26, 27,and 28 have mutually different values. By adjusting the impurityconcentration of the semiconductor layer 53 of each of the temperaturecompensation diodes 26, 27, and 28 and the impurity concentration of thesemiconductor layer 53 of the APD 11, the difference voltage between thebreakdown voltage of the APD 11 and the breakdown voltage of each of thetemperature compensation diodes 26, 27, and 28 is adjusted. As amodification example of the present embodiment, the difference voltagemay be adjusted depending on the circuit configuration. The differencevoltage may be adjusted by applying an external voltage to the terminal25. In the case of these modification examples, the breakdown voltage ofthe APD 11 and the breakdown voltage of any one of the temperaturecompensation diodes 26, 27, and 28 may be equal to each other. Thedifference voltage may be adjusted by combining these plurality ofmethods.

In the present embodiment, the impurity concentration of thesemiconductor layer 53 of each of the temperature compensation diodes26, 27, and 28 is higher than the impurity concentration of thesemiconductor layer 53 of the APD 11. As a result, the breakdown voltageof the APD 11 is higher than the breakdown voltage of each of thetemperature compensation diodes 26, 27, and 28 by “ΔV”. The threetemperature compensation diodes 26, 27, and 28 have mutually differentbreakdown voltages. The three temperature compensation diodes 26, 27,and 28 are designed to obtain mutually different gains. “ΔV” iscalculated for each of the temperature compensation diodes 26, 27, and28 according to Equation (4), and the impurity concentration of thesemiconductor layer 53 of each of the temperature compensation diodes26, 27, and 28 is designed according to each calculated “ΔV”. Whencalculating “ΔV” for each of the temperature compensation diodes 26, 27,and 28, the same value is substituted into “a”. Similarly, whencalculating “ΔV” for each of the temperature compensation diodes 26, 27,and 28, the same value is substituted into “b”.

In the light detection device 1, since a breakdown voltage of each ofthe temperature compensation diodes 26, 27, and 28 is applied, thebreakdown voltage is applied to the APD 11 as a bias voltage. In thepresent embodiment, one of the breakdown voltages of the plurality oftemperature compensation diodes 26, 27, and 28 is applied to the APD 11as a bias voltage. Which of the breakdown voltages of the plurality oftemperature compensation diodes 26, 27, and 28 is to be applied to theAPD 11 as a bias voltage is controlled by the setting unit 40.

Next, the operation of the light detection device according to thepresent embodiment will be described.

In the present embodiment, the terminal 22 is connected to thesemiconductor layer 54 of the P⁺ type, and the semiconductor layer 54 isconnected to the semiconductor layer 55 of the P⁺ type. Therefore, theanodes of the APD 11 and the temperature compensation diodes 26, 27, and28 are connected to the terminal 22 in parallel with each other. As aresult, a negative potential is applied to the anodes of the APD 11 andthe temperature compensation diodes 26, 27, and 28 by the power supplyunit 4.

The circuit unit 3 causes one of the plurality of temperaturecompensation diodes 26, 27, or 28 to break down. The setting unit 40selects a temperature compensation diode to be operated among theplurality of temperature compensation diodes 26, 27, and 28 using theswitch 41. The setting unit 40 selects a temperature compensation diodeto apply a breakdown voltage to the APD 11 as a bias voltage, byswitching ON/OFF of the switch 41. The setting unit 40 selects atemperature compensation diode to be used for controlling the biasvoltage among the plurality of temperature compensation diodes 26, 27,and 28 so that “ΔV” calculated by substituting the gain to be set in theAPD 11 into “M_(d)” in Equation (4) becomes a difference voltage.

The breakdown voltage of the selected temperature compensation diodecorresponds to a potential difference between the potential applied tothe terminal 25 corresponding to the temperature compensation diode andthe potential applied to the terminal 22. Therefore, a potentialcorresponding to the breakdown voltage of the selected temperaturecompensation diode is applied to the anode of the APD 11. As a result, avoltage corresponding to the breakdown voltage of the selectedtemperature compensation diode is applied to the APD 11 as a biasvoltage.

In the present embodiment, when operating the temperature compensationdiode 28, the setting unit 40 sets all the temperature compensationdiodes 26, 27, and 28 to the state capable of being electricallyenergized. That is, the setting unit 40 turns on all of the switches 41connected to the plurality of terminals 25. In this case, thetemperature compensation diode 28 has a lowest breakdown voltage amongthe temperature compensation diodes 26, 27, and 28 set in the statecapable of being electrically energized, so that the temperaturecompensation diode 28 operates. That is, the breakdown voltage of thetemperature compensation diode 28 is applied to the APD 11 as a biasvoltage.

When operating the temperature compensation diode 27, the setting unit40 sets the temperature compensation diodes 26 and 27 to the statecapable of being electrically energized, and sets the temperaturecompensation diode 28 to the state incapable of being electricallyenergized. In the present embodiment, the setting unit 40 turns on theswitch 41 connected to the terminal 25 corresponding to the temperaturecompensation diode 27, and turns off the switch 41 connected to theterminal 25 corresponding to the temperature compensation diode 28.Since the switch 41 is not connected to the terminal 25 corresponding tothe temperature compensation diode 26, the temperature compensationdiode 26 is in the state capable of being electrically energized. Inthis case, the temperature compensation diode 27 has a lowest breakdownvoltage between the temperature compensation diodes 26 and 27 set in thestate capable of being electrically energized, so that the temperaturecompensation diode 27 operates. That is, the breakdown voltage of thetemperature compensation diode 27 is applied to the APD 11 as a biasvoltage.

When operating the temperature compensation diode 26, the setting unit40 sets the temperature compensation diode 26 to the state capable ofbeing electrically energized, and sets the temperature compensationdiodes 27 and 28 to the state incapable of being electrically energized.In the present embodiment, the setting unit 40 turns off the switch 41connected to the terminal 25 corresponding to the temperaturecompensation diodes 27 and 28. Since the switch 41 is not connected tothe terminal 25 corresponding to the temperature compensation diode 26,the temperature compensation diode 26 is set in the state capable ofbeing electrically energized. In this case, the temperature compensationdiode 26 set in the state capable of being electrically energizedoperates. That is, the breakdown voltage of the temperature compensationdiode 26 is applied to the APD 11 as a bias voltage.

According to the operation described above, the gain of the APD 11 isselected by the setting unit 40. FIG. 6 is a graph illustrating theoutput characteristics of the APD 11 according to the setting by thesetting unit 40. In FIG. 6, the vertical axis indicates the outputvoltage of the APD 11, and the horizontal axis indicates time. Eachpiece of data 71, 72, and 73 indicates the output characteristics of theAPD 11 when pulsed light with the strength equal to each other entersthe APD 11. The data 71 indicates the output characteristics of the APD11 in a state in which the temperature compensation diode 26 isoperating. The data 72 indicates the output characteristics of the APD11 in a state in which the temperature compensation diode 27 isoperating. The data 73 indicates the output characteristics of the APD11 in a state in which the temperature compensation diode 26 isoperating.

As illustrated in FIG. 6, the output peak of the APD 11 in a state inwhich the temperature compensation diode 26 is operating is larger thanthe output peak of the APD 11 in a state in which the temperaturecompensation diode 27 is operating. The output peak of the APD 11 in astate in which the temperature compensation diode 27 is operating islarger than the output peak of the APD 11 in a state in which thetemperature compensation diode 28 is operating. Thus, it has beenconfirmed that an operating temperature compensation diode 26, 27, 28 isswitched by the setting unit 40, so that the gain of the APD 11 isselected.

In the present embodiment, the setting unit 40 sets the temperaturecompensation diode 26 to a state capable of being electricallyenergized, regardless of whether or not the temperature compensationdiode 28 is in a state capable of being electrically energized. In astate incapable of electrically energizing the temperature compensationdiode 28, the setting unit 40 switches by the switch 41 between a statecapable of electrically energizing the temperature compensation diode 27and a state incapable of electrically energizing the temperaturecompensation diode 27. Hereinafter, a case where the temperaturecompensation diode 28 is selected as a temperature compensation diode tobe operated by the setting unit 40 will be described as an example.

In the present embodiment, since a combination of the electromotiveforce generation unit 31 and the current limiting unit 32 is connectedto the terminal 22, the breakdown voltage of the selected temperaturecompensation diode 28 is applied to the terminal 22. In the presentembodiment, the output voltage of the electromotive force generationunit 31 is equal to or higher than the operating voltage of the APD 11.In other words, the output voltage of the electromotive force generationunit 31 is equal to or higher than the upper limit of the temperaturechange of the breakdown voltage of each temperature compensation diode26, 27, 28. For example, the output voltage of the electromotive forcegeneration unit 31 is 300 V or higher. The current limiting unit 32 isconfigured to include, for example, a current mirror circuit or aresistor.

The gain of the APD 11 can be arbitrarily set according to the breakdownvoltage difference between the selected temperature compensation diode28 and the APD 11. When the gain of the APD 11 is set to an optimalmultiplication factor Mopt having a high S/N ratio, the detectionaccuracy can be improved.

In the present embodiment, the anodes of the APD 11 and the temperaturecompensation diodes 26, 27, and 28 are integrally formed in thesemiconductor layer 55. For example, when the potential applied to theterminal 25 is 0 V and the breakdown voltage of the selected temperaturecompensation diode 28 is 130 V under an ambient temperature of 25° C., apotential of −130 V is applied to the anode of the APD 11. Therefore,when the breakdown voltage of the APD 11 is 150 V under an ambienttemperature of 25° C., the APD 11 operates in a state in which thepotential difference between the anode and the cathode is lower by 20 Vthan the breakdown voltage.

As described above, the APD 11 and the temperature compensation diodes26, 27, and 28 have the same temperature characteristics with respect tothe relationship between the gain and the bias voltage. Therefore, aslong as the selected temperature compensation diode 28 is in a breakdownstate, the APD 11 operates while maintaining the gain of a case in whicha bias voltage lower by 20 V than the breakdown voltage is applied underan ambient temperature of 25° C. In other words, in the light detectiondevice 1, a voltage that causes the selected temperature compensationdiode 28 to break down is applied to the temperature compensation diode28, so that temperature compensation is provided for the gain of the APD11.

Next, the operational effects of the light detection devices in theabove-described embodiment and modification examples will be described.Conventionally, when manufacturing a light detection device including anAPD and a temperature compensation diode having the same temperaturecharacteristics, it has been necessary to select and combine APDs havingdesired temperature characteristics with respect to the relationshipbetween the gain and the bias voltage. For this reason, it has beendifficult to reduce the cost. In this regard, in the light detectiondevice 1, the APD 11 and the temperature compensation diodes 26, 27, and28 are independently formed on the same semiconductor substrate 50. Inthis case, the temperature compensation diodes 26, 27, and 28 and theAPD 11 having the same temperature characteristics over a widetemperature range with respect to the gain and the bias voltage areformed more easily and accurately than in a case where the temperaturecompensation diodes 26, 27, and 28 and the APD 11 are formed on mutuallydifferent semiconductor substrates. Therefore, temperature compensationfor the gain of the APD 11 can be provided while suppressing themanufacturing cost.

The semiconductor substrate 50 includes the semiconductor region 51 ofthe first conductive type. Each of the APD 11 and the temperaturecompensation diodes 26, 27, and 28 includes the semiconductor layer 52and the semiconductor layer 53. In the semiconductor substrate 50, thesemiconductor layer 52 is a second conductive type. The semiconductorlayer 53 is a first conductive type having a higher impurityconcentration than the semiconductor region 51. The semiconductor layer53 is located between the semiconductor region 51 and the semiconductorlayer 52. As described above, the temperature compensation diodes 26,27, and 28 have the same configuration as the APD 11. Therefore, it ispossible to easily form the temperature compensation diodes 26, 27, and28 whose temperature characteristics with respect to the gain and thebias voltage are very similar to that of the APD 11.

In the semiconductor substrate 50, the impurity concentration in thesemiconductor layer 53 of each of the temperature compensation diodes26, 27, and 28 is higher than the impurity concentration in thesemiconductor layer 53 of the APD 11. In this case, in the lightdetection device 1, for example, the breakdown voltage of the APD 11 ishigher than the breakdown voltage of each of the temperaturecompensation diodes 26, 27, and 28. As a result, temperaturecompensation for the gain of the APD 11 arranged to operate in linearmode is provided.

In the light detection device 1, the difference voltage to obtain thedesired gain is determined by substituting the slope of the regressionline having “(1/M)×(dM/dV_(r))” as an objective variable and “M” as anexplanatory variable into “a” in Equation (4) and substituting theintercept of the regression line into “b” in Equation (4). Therefore, adesired gain can be obtained very easily without strictly consideringthe ambient temperature.

The temperature compensation unit 15 includes the temperaturecompensation diodes 26, 27, and 28. The temperature compensation unit 15applies a voltage corresponding to the breakdown voltage, which isapplied to any one of the temperature compensation diodes 26, 27, and28, to the APD 11 as a bias voltage. For example, when the temperaturecompensation diode 28 is in a breakdown state, the difference voltagecorresponds to a subtraction value obtained by subtracting a voltagecorresponding to the breakdown voltage of the temperature compensationdiode 28 from the breakdown voltage of the APD 11. Therefore, it ispossible to derive “ΔV” to obtain the desired gain and design theimpurity concentrations of the APD 11 and the temperature compensationdiodes 26, 27, and 28 so that the subtraction value becomes “ΔV”.

The light detection device 1 includes the setting unit 40 and the wiringunit 21. The setting unit 40 sets the temperature compensation unit 15according to the gain to be set in the APD 11. The wiring unit 21electrically connects the temperature compensation unit 15 and the APD11 to each other. The plurality of temperature compensation diodes 26,27, and 28 have mutually different breakdown voltages. The wiring unit21 applies, to the APD 11 as a bias voltage, a voltage corresponding tothe breakdown voltage of each of the temperature compensation diodes 26,27, and 28. The setting unit 40 sets a temperature compensation diode tobe used for controlling the bias voltage among the plurality oftemperature compensation diodes 26, 27, and 28 so that “ΔV” calculatedby substituting the gain to be set in the APD 11 into “M_(d)” inEquation (4) becomes a difference voltage. As a result, a temperaturecompensation diode to be used for controlling the bias voltage among theplurality of temperature compensation diodes 26, 27, and 28 is set bythe setting unit 40. Therefore, a gain desired according to thesituation can be obtained very easily without strictly considering theambient temperature. In other words, it is possible to easily switch adesired gain and obtain the desired gain in a stable manner with respectto temperature.

The circuit unit 3 electrically connects the APD 11 and the temperaturecompensation diodes 26, 27, and 28 to the terminal 22 in parallel witheach other. In this configuration, when any one of the plurality oftemperature compensation diodes 26, 27, and 28 is in a breakdown state,the breakdown voltage of the temperature compensation diode in thebreakdown state is applied to the APD 11 as a bias voltage. As a result,the difference voltage between the breakdown voltage of the APD 11 andthe bias voltage applied to the APD 11 is set, and the APD 11 has a gaincorresponding to the difference voltage. Therefore, according to atemperature compensation diode that breaks down, a gain desiredaccording to the situation can be obtained in a stable manner withrespect to temperature in the APD 11.

The circuit unit 3 includes at least one switch 41. The switches 41 areelectrically connected to the corresponding temperature compensationdiodes 27 and 28. The switches 41 switch between a state capable ofelectrically energizing the corresponding temperature compensationdiodes 27 and 28 and a state incapable of electrically energizing thecorresponding temperature compensation diodes 27 and 28. The pluralityof temperature compensation diodes 26, 27, and 28 include thetemperature compensation diode 26 and the temperature compensation diode28. The temperature compensation diode 26 has a higher breakdown voltagethan the temperature compensation diode 28. The switch 41 iselectrically connected to the temperature compensation diode 28. In thiscase, when the temperature compensation diode 28 is set to a statecapable of being electrically energized by the switch 41, thetemperature compensation diode 28 preferentially breaks down even if thetemperature compensation diode 26 is in a state capable of beingelectrically energized. In this manner, it is possible to switch a gaindesired according to the situation in the APD 11 with simple control.

At least one switch 41 is connected to the corresponding terminal 25. Ahigh voltage is applied between the electrode 29 a of each of thetemperature compensation diodes 26, 27, and 28 and the APD 11.Therefore, a control in a case where the switch 41 is electricallyconnected to the electrode 29 b through the terminal 25 can be easierthan that in a case where the switch 41 is disposed between theelectrode 29 a and the APD 11.

The circuit unit 3 is configured to set the temperature compensationdiode 26 into a state capable of being electrically energized regardlessof whether or not to be capable of electrically energizing thetemperature compensation diode 28. In this case, even if the temperaturecompensation diode 28 is damaged or a local temperature change occurs inthe vicinity where the temperature compensation diode 28 is disposed,the temperature compensation diode 26 breaks down. Therefore, the flowof a large current to the APD 11 is prevented, and the failure of thelight detection device 1 is prevented.

The plurality of temperature compensation diodes 26, 27, and 28 furtherinclude the temperature compensation diode 27. The temperaturecompensation diode 27 has a breakdown voltage that is higher than thebreakdown voltage of the temperature compensation diode 28 and lowerthan the breakdown voltage of the temperature compensation diode 26. Theswitch 41 is electrically connected to the temperature compensationdiode 27. In a state incapable of electrically energizing thetemperature compensation diode 28, the circuit unit 3 is configured toswitch by the switch 41 between a state capable of electricallyenergizing the temperature compensation diode 27 and a state incapableof electrically energizing the temperature compensation diode 27. Inthis case, in a state capable of electrically energizing the temperaturecompensation diode 28, the temperature compensation diode 28 breaksdown. In a state incapable of electrically energizing the temperaturecompensation diode 28, the temperature compensation diode 27 breaks downwhen the temperature compensation diode 27 is set in a state capable ofbeing electrically energized. In a state incapable of electricallyenergizing the temperature compensation diode 28, the temperaturecompensation diode 26 breaks down when the temperature compensationdiode 27 is set in a state incapable of being electrically energized. Inthis manner, it is possible to switch a gain desired according to thesituation in the APD 11 with simple control.

Next, an example of a method for manufacturing a light detection devicewill be described with reference to FIG. 7. FIG. 7 is a flowchartillustrating a method for manufacturing the semiconductor substrate 50in the light detection device 1.

First, a semiconductor wafer is prepared (process S1). The semiconductorwafer is a substrate before being processed as the semiconductorsubstrate 50, and has main surfaces 50 a and 50 b facing each other. Thesemiconductor wafer has a first conductive type semiconductor regioncorresponding to the semiconductor region 51. The semiconductor regionis provided on the main surface 50 a side of the semiconductor wafer,and forms the entire main surface 50 a. For example, the semiconductorregion of the semiconductor wafer is P⁻ type. In the present embodiment,the semiconductor layer 55 of the first conductive type, which has animpurity concentration higher than the semiconductor region of thesemiconductor wafer, is formed in the semiconductor wafer by addingimpurities from the main surface 50 b side. For example, thesemiconductor layer 55 is P⁺ type.

Subsequently, the difference voltage between the breakdown voltage ofthe APD 11 and the bias voltage applied to the APD 11 is determined. Thedetermination method is as follows.

First, the slope and intercept of the regression line, which has“(1/M)×(dM/dV_(r))” as an objective variable and “M” as an explanatoryvariable in the data indicating the correlation between the bias voltageapplied to the APD and the gain of the APD, are obtained (process S2).Here, “V_(r)” is a bias voltage applied to the APD, and “M” is the gainof the APD to which the bias voltage is applied. The above data used inprocess S2 corresponds to a separate body having the same material andstructure as the APD 11.

Then, the difference voltage to obtain the desired gain is determined byusing the result obtained in process S2 and Equation (4) (process S3).The above difference voltage corresponds to “ΔV” calculated bysubstituting the obtained slope into “a” in Equation (4), substitutingthe obtained intercept into “b” in Equation (4), and substituting thedesired gain to be set in the APD 11 into “M_(d)” in Equation (4). Inthe present embodiment, a plurality of values different from each otherare determined as a gain to be set in the APD 11, and a plurality ofdifference voltages described above are determined for these values. Aplurality of “ΔV” calculated by substituting a plurality of valuesdifferent from each other into “M_(d)” in Equation (4) are determined asthe difference voltages corresponding to the plurality of values.

Subsequently, as first ion implantation process (process S4), impurityions are implanted to the main surface 50 a side using an ionimplantation method to add impurities, forming the second conductivetype semiconductor layer 52 and the first conductive type semiconductorlayers 53 and 54. For example, the semiconductor layer 52 is N⁺ type,the semiconductor layer 53 is P type, and the semiconductor layer 54 isP⁺ type. In the present embodiment, the semiconductor layer 52 is formedby implanting second conductive type impurity ions into differentportions spaced apart from each other in one ion implantation process.The semiconductor layer 53 is formed by implanting first conductive typeimpurity ions after the semiconductor layer 52 is formed. Thesemiconductor layer 53 may be formed by implanting first conductive typeimpurity ions before the semiconductor layer 52 is formed.

The semiconductor layers 52 and 53 are formed at locations overlappingeach other when viewed from the direction perpendicular to the mainsurface 50 a. The semiconductor layer 53 is formed by implanting firstconductive type impurities at a location deeper than the semiconductorlayer 52 when viewed from the main surface 50 a side. The semiconductorlayers 52 and 53 are formed at a plurality of portions spaced apart fromeach other when viewed from the direction perpendicular to the mainsurface 50 a, in a region serving as one semiconductor substrate 50. Theplurality of portions include a portion where the APD 11 is disposed anda portion where each of the temperature compensation diodes 26, 27, and28 is disposed. In the first ion implantation process, second conductivetype impurities are added to each portion so that the impurityconcentration of the semiconductor layer 52 is the same. Similarly,first conductive type impurities are added to each portion so that theimpurity concentration of the semiconductor layer 53 is the same.

Subsequently, as second ion implantation process (process S5),impurities are further added only to the semiconductor layer 53 in someof the above-described plurality of portions by using an ionimplantation method. In the present embodiment, the first conductivetype impurities are further implanted into the semiconductor layer 53only in a portion where each of the temperature compensation diodes 26,27, and 28 is disposed. Accordingly, in the light detection device 1,the impurity concentration in the semiconductor layer 53 of each of thetemperature compensation diodes 26, 27, and 28 is higher than theimpurity concentration in the semiconductor layer 53 of the APD 11. Inthis case, the light detection device 1 is configured such that thebreakdown voltage of the APD 11 is higher than the breakdown voltage ofeach of the temperature compensation diodes 26, 27, and 28.

The amount of the first conductive type impurities implanted into thesemiconductor layer 53 of each of the temperature compensation diodes26, 27, and 28 in processes S4 and S5 depends on the difference voltagedetermined in process S3. In the present embodiment, the amount of thefirst conductive type impurities implanted into the semiconductor layer53 of the temperature compensation diode 28 is larger than the amount ofthe first conductive type impurities implanted into the semiconductorlayer 53 of the temperature compensation diode 27. Therefore, thebreakdown voltage of the temperature compensation diode 27 is configuredto be larger than the breakdown voltage of the temperature compensationdiode 28. The amount of the first conductive type impurities implantedinto the semiconductor layer 53 of the temperature compensation diode 27is larger than the amount of the first conductive type impuritiesimplanted into the semiconductor layer 53 of the temperaturecompensation diode 26. Therefore, the breakdown voltage of thetemperature compensation diode 26 is configured to be larger than thebreakdown voltage of the temperature compensation diode 27.

In the second ion implantation process, the first conductive typeimpurities may be further implanted into the semiconductor layer 53 onlyin a portion where the APD 11 is disposed, not in a portion where eachof the temperature compensation diodes 26, 27, and 28 is disposed. Inthis case, in the light detection device 1, the impurity concentrationin the semiconductor layer 53 of each of the temperature compensationdiodes 26, 27, and 28 is lower than the impurity concentration in thesemiconductor layer 53 of the APD 11. In the light detection device 1 inthis case, the breakdown voltage of the APD 11 is configured to be lowerthan the breakdown voltage of each of the temperature compensationdiodes 26, 27, and 28.

By the processes described above, the semiconductor substrate 50 of thelight detection device 1 is formed. Processes S2 and S3 may be performedbefore process S1 or after process S4. In the present embodiment, thesemiconductor layers 52, 53, and 54 are formed from the state in whichthe semiconductor layer 55 has already been formed. However, thesemiconductor layer 55 may be formed after the semiconductor layers 52,53, and 54 are formed.

In the manufacturing method described above, the semiconductor layer 52and the semiconductor layer 53 are formed in each portion by implantingions into a plurality of different portions. Thereafter, ions arefurther implanted into the semiconductor layer 53 in some of theportions. Therefore, the plurality of temperature compensation diodes26, 27, and 28 and the APD 11 each of which is set to the desiredbreakdown voltage can be easily manufactured while having the sametemperature characteristics with respect to the gain and the biasvoltage. In this case, the gain of the APD 11 can be arbitrarily setaccording to the difference voltage between the breakdown voltage ofeach of the temperature compensation diodes 26, 27, and 28 and thebreakdown voltage of the APD 11. Therefore, when each of the temperaturecompensation diodes 26, 27, and 28 and the APD 11 is set to the desiredbreakdown voltage, the detection accuracy can be improved. For example,when the gain of the APD 11 is set to the optimal multiplication factorMopt having a high S/N ratio according to the above difference voltage,the detection accuracy can be improved. Thus, in the manufacturingmethod described above, temperature compensation for the gain of the APD11 is provided while suppressing the manufacturing cost, and thedetection accuracy can be improved.

In the difference voltage determination method described above, theslope and intercept of the regression line, which has“(1/M)×(dM/dV_(r))” as an objective variable and “M” as an explanatoryvariable, are obtained. By substituting the obtained slope into “a” inEquation (4) and substituting the obtained intercept into “b” inEquation (4), the difference voltage to obtain the desired gain isdetermined. Therefore, the difference voltage to obtain the desired gainis determined very easily without strictly considering the ambienttemperature.

In the determination method described above, a plurality of “ΔV”calculated by each substituting a plurality of different values as gainsto be set in the APD 11 into “M_(d)” in Equation (4) are determined asdifference voltages corresponding to the plurality of values. Therefore,the plurality of difference voltages corresponding to the plurality ofvalues are determined very easily without strictly considering theambient temperature.

While the embodiment of the present invention and the modificationexamples have been described above, the present invention is notnecessarily limited to the embodiment and the modification examplesdescribed above, and various changes can be made without departing fromthe scope of the present invention.

In the present embodiment, the configuration in which the so-calledreach-through type APD 11 is arranged to operate in linear mode has beendescribed. The light detection device 1 may have a configuration inwhich the reverse type APD 11 is arranged to operate in linear mode.

In the present embodiment, the light detection device 1 including theelectromotive force generation unit 31, the current limiting unit 32,the bias voltage stabilization unit 33, and the setting unit 40 has beendescribed. However, the light detection device according to the presentembodiment may have a configuration in which at least one of theelectromotive force generation unit 31, the current limiting unit 32,the bias voltage stabilization unit 33, or the setting unit 40 is notincluded. In this case, an external device connected to the lightdetection device may function as the electromotive force generation unit31, the current limiting unit 32, the bias voltage stabilization unit33, or the setting unit 40. The light detection device 1 may include asignal reading circuit (not illustrated).

In the present embodiment, the configuration has been described in whichthe switch 41 is connected to the terminal 25 of the light detectionunit 20 and the switch 41 is controlled by the setting unit 40. However,the switch 41 may be disposed inside the light detection unit 20.

In the present embodiment, the terminals 22, 23, 24, and 25 have beendescribed as pad electrodes. However, the terminals 22, 23, 24, and 25may be configured by the semiconductor in the semiconductor substrate50.

The switch 41 for switching the electrical connection between each ofthe temperature compensation diodes 26, 27, and 28 and the APD 11 may bedisposed in the wiring unit 21, and ON/OFF of the switch 41 in thewiring unit 21 may be controlled by the setting unit 40. Also in thiscase, the setting unit 40 controls the bias voltage applied to the APD11. Since a high voltage is applied between the APD 11 and each of thetemperature compensation diodes 26, 27, and 28, the switch 41 connectedto the terminal 25 is controlled more easily than in a case where theswitch disposed in the wiring unit 21 is controlled.

The temperature compensation unit 15 may include a plurality oftemperature compensation diodes having the same breakdown voltage.According to this configuration, even if a part of the temperaturecompensation diode is damaged or a local temperature change occurs inthe vicinity where a part of the temperature compensation diode isdisposed, the normal operation of the light detection device 1 can berealized.

REFERENCE SIGNS LIST

-   -   1: light detection device, 3: circuit unit, 11: APD, 22, 25:        terminal, 26, 27, 28: temperature compensation diode, 41:        switch.

1. A light detection device, comprising: an avalanche photodiode; aplurality of temperature compensation diodes each of which includesfirst and second electrodes and which have mutually different breakdownvoltages lower than a breakdown voltage of the avalanche photodiode; anda circuit unit configured to cause any one of the plurality oftemperature compensation diodes to break down, wherein the circuit unitincludes a plurality of first terminals, which are respectivelyconnected to the second electrodes of the temperature compensationdiodes different from each other, and a second terminal electricallyconnected to the avalanche photodiode and the first electrode of each ofthe temperature compensation diodes.
 2. The light detection deviceaccording to claim 1, wherein the circuit unit includes at least oneswitch electrically connected to a corresponding temperaturecompensation diode and configured to switch between a state capable ofelectrically energizing the corresponding temperature compensation diodeand a state incapable of electrically energizing the correspondingtemperature compensation diode, the plurality of temperaturecompensation diodes include a first temperature compensation diode and asecond temperature compensation diode having a higher breakdown voltagethan the first temperature compensation diode, and the switch iselectrically connected to the first temperature compensation diode. 3.The light detection device according to claim 2, wherein the at leastone switch is connected to a corresponding first terminal.
 4. The lightdetection device according to claim 2, wherein the circuit unit isconfigured to be set to a state capable of electrically energizing thesecond temperature compensation diode regardless of whether or not to becapable of electrically energizing the first temperature compensationdiode.
 5. The light detection device according to claim 4, wherein theplurality of temperature compensation diodes further include a thirdtemperature compensation diode having a breakdown voltage higher than abreakdown voltage of the first temperature compensation diode and lowerthan a breakdown voltage of the second temperature compensation diode,the switch is electrically connected to the third temperaturecompensation diode, and the circuit unit is configured to switch, by theswitch, between a state capable of electrically energizing the thirdtemperature compensation diode and a state incapable of electricallyenergizing the third temperature compensation diode, in a stateincapable of electrically energizing the first temperature compensationdiode.